pgsql: Fix compiler warning on typedef redeclaration

Fix compiler warning on typedef redeclaration

    bulk_write.c:78:3: error: redefinition of typedef 'BulkWriteState' is a C11 feature
[-Werror,-Wtypedef-redefinition]
    } BulkWriteState;
      ^
    ../../../../src/include/storage/bulk_write.h:20:31: note: previous definition is here
    typedef struct BulkWriteState BulkWriteState;
                                  ^
    1 error generated.

Per buildfarm animals 'sifaka' and 'longfin'.

Discussion: https://www.postgresql.org/message-id/9e1f63c3-ef16-404c-b3cb-859a96eaba39@iki.fi

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/d360e3cc60e37d6d41ee78acf7bd5cb313b978c1

Modified Files
--------------
src/backend/storage/smgr/bulk_write.c | 4 ++--
src/include/storage/bulk_write.h      | 1 +
2 files changed, 3 insertions(+), 2 deletions(-)